Long Term Evolution (LTE) relates to wireless cellular communication of Fourth Generation (4G). With LTE, Orthogonal Frequency Division Multiplexing (OFDM) is adopted. A time-frequency resource formed by a subcarrier and an OFDM symbol constitutes a wireless physical time-frequency resource of a LTE system. Existing OFDM has been applied widely in wireless communication. As a cyclic prefix (CP) is adopted. With a CP-OFDM system, an issue caused by a multipath delay may be well dealt with, and a frequency selective channel is divided into a set of parallel flat channels. This well simplifies channel estimation, and leads to a high precision in channel estimation. However, performance of a CP-OFDM system is sensitive to a time offset and a frequency offset between neighboring sub-bands, mainly due to a large spectrum leakage of the system. As a result, inter-sub-band interference may occur. An existing LTE system uses a protecting interval on a frequency domain. However, this lowers a spectral efficiency. Therefore, some new technologies may have to be adopted to suppress out-of-band leakage.
Major companies are starting research on Fifth Generation (5G) wireless communication, with out-of-band leakage suppression as a focus of 5G research. With new multi-carrier solutions according to some recent documents, i.e., Filter Bank Multicarrier (FBMC) and Generalized Frequency Division Multiplexing (GFDM), out-of-band leakage may be suppressed. However, FBMC and GFDM may be incompatible with LTE. Moreover, there may be an issue such as channel estimation, integration with Multiple Input Multiple Output (MIMO), etc. Although Filtered OFDM (F-OFDM) and Universal Filtered Multicarrier (UFMC) technology according to other documents are somewhat compatible with LTE, out-of-band leakage may not be suppressed very well with F-OFDM and UFMC. In addition, in-band subcarriers may still have to be strictly synchronized. That is, the in-band subcarriers may still be sensitive to a time offset and a frequency offset in a sub-band. Moreover, performance of demodulation at a receiving end may somewhat decline.
Therefore, a good solution capable of well suppressing out-of-band leakage while maintaining as much compatibility with an LTE system as possible may have to be proposed.